The aim of this work is to elucidate the relations between structure and function in hemoglobin. In the context of this aim the properties of gelation of hemoglobin S are being studied with the aims of 1) characterzing the physical and stereochemical properties of gelation, 2) inhibiting gelation and 3) using the gelation process as a marker for quaternary conformational (allosteric) isomerization. In respect to this last aim, gelation of methemoglobin S has been examined by a newly developed equilibrium ultracentrifugation method. Methemoglobin S from which organic phosphates have been removed (stripped) gels at high hemoglobin concentration. When inositol hexaphosphate (IHP) is added it gels at about 35 o/o. Since gelation requires the T ("deoxy") quaternary structure this indicates that this derivative, usually present in R ("oxy") conformation, exists in R-T equilibrium in solution, confirming conclusions drawn by less direct methods. Since gelling concentrations are greater than for deoxyhemoglobin S the equilibrium lies between the extremes exhibited by deoxyhemoglobin (T) and oxyhemoglobin (R). The gelation of methemoglobin S is favored by lowering pH toward 6. This and other physical chemical approaches will be employed to gain further information on conformational isomerizations.